One of the important parameters having an influence on the performance of an MOS integrated circuit, which includes insulated-gate field-effect transistors (IGFETs), is the value of the threshold voltage of these transistors. The performance in question can be for example their operational limit as a function of their supply voltage or of the temperature.
In the manufacture of circuits of this type it is relatively easy to minimize the spread of the values of the threshold voltage of transistors located on the same chip, this spread being essentially due to the "flat-band" voltage. The situation is, however, not the same so far as the spread of values between transistors located on different chips is concerned, particularly if, although obtained by the same manufacturing operation, these chips belong to different batches. This is why the attempt has been made to design systems which regulate, by electronic means, the threshold voltage of the transistors of an integrated circuit to a well-defined value.
MOS integrated circuits are already known which embody such regulating means operating by the biasing of the substrate of the transistors concerned. Reference may be made in this connection to U.S. Pat. No. 3,806,741 and to a publication by Eugene M. Blaser, William M. Chu and George Sonoda entitled "Substrate Load Gate Voltage Compensation" (Digest of Technical Papers, 1976 IEEE International Solid-State Circuits Conference, pp. 5657).
In these systems, the regulation is achieved by comparing a reference voltage, derived from the supply voltage, with the threshold voltage of a transistor; the difference of these voltages, constituting an error signal, controls an inverter which amplifies it. The amplified signal modulates a pulse generator whose output voltage is rectified and constitutes the biasing voltage of the substrate, which is adjusted in such a way that the error signal is close to zero.
If it is desired to employ such a system to perform regulation of the threshold voltage of transistors of an MOS integrated circuit of a quartz wristwatch, for example, problems of energy consumption have to be faced.
In a portable unit such as a wristwatch, the space available for the constituent elements is very restricted. Thus, the capacity of the energy source which it contains and which is usually a cell of small dimensions is also very limited. To ensure self-sufficiency of long duration, e.g. of several years, it is necessary that the consumption of the complete electronic system be very small.
Electronic circuits of low energy consumption have already been proposed and permit a rationalized manner of manufacture, embodied in the form of MOS integrated circuits containing only transistors of a single conduction type and capacitors. Such circuits are described, for instance, in U.S. Pat. Nos. 3,956,714, 3,932,773 and 3,983,411. They are advantageously embodied by integration in substrates of relatively low doping. This reduces, on the one hand, the effect of variations of the threshold voltage of their transistors as a function of the source potential thereof (body effect) and, on the other hand, the parasitic capacitance of the different diffused zones with respect to the substrate.
Given that the threshold voltage of MOS transistors varies only with the square root of the biasing voltage applied between their source and the substrate, regulation of the threshold voltage by means of such a bias in a relatively large domain, e.g. several tenths of a volt, requires that the biasing voltage of such a substrate, weakly doped, should attain values which are a multiple of the cell voltage (1.3 to 1.5 volt) usual in electronic wristwatches.